Sound-proofing using moth wings

Sound is everywhere. The world is filled with not just the sounds that we can hear, but also those that we can’t. Now, an interesting news: the future sound-proof walls may have coatings of moth wings! Engineer Marc W Holderied and his colleagues at Bristol University, UK have discovered that the wings of a moth could be great soundproofing material.

Music is good to hear in a sound-proof room. Porous and fibrous materials such as wood panels or Plaster of Paris boards, though good in soundproofing, add to the thickness of the wall. This is because the sound-absorbing material has to be thicker than the maximum wavelength of the sounds pounding on it.

So engineers have been on the lookout for a thinner, perfect soundproofing material that can absorb sounds of wavelength greater than the material’s thickness. Of course, there are metamaterials, purposefully created special substances, that can absorb sound effectively. Some of them can absorb sound waves that are 200 times larger than their thickness. But, they cover only a narrow range of sounds. Holderied’s team believes that moth wings are natural sound absorbers for a broader range of sounds.

Though not containing any dye, the thin scales on moth and butterfly wings can create a myriad of colours. They are also known to be sound absorbers, especially of low-frequency sounds that constitute much of what we know as murmur. Some mothwings can absorb sounds with wavelengths a hundred times longer than their thickness. This helps to hide from their predators such as bats using ultrasound to find and hunt moths.

Holderied’s team used Antheraea pernyi, a relative of the Muga silkmoth whose wings are sufficiently large to cut into several discs. They were placed on an aluminium foil disc and sounds of various frequencies were shot at it. Their echoes were caught on a detector and intensity measured.

The team wanted to find out how and why this dampening happens. So, they tried measuring echoes after the scales coating the wings. The results were surprising. With or without scales the wings reduced echoes. But when scales were removed, echoes were reduced only when the scale-less surface of the wing was towards the sound source. However, a quarter to half of the echoes were reduced with wings on the aluminium disc. Though it is not clear how moth wings absorb sound, Holderied’s team believes that it can help produce a good thin coating to soundproof surfaces.

The findings have been published in the recent issue of Proceedings of Royal Society A.

Kollegala Sharma